1. Field of the Invention
The present invention relates to a method of manufacturing an image display device of a flat shape, which has oppositely arranged substrates.
2. Description of the Related Art
Recently, various types of image display devices have been developed as the next generation image display device, light in weight and flat in shape, which will supersede a cathode ray tube (referred to as a CRT). Examples of those image display devices are a liquid crystal display (referred to as an LCD) which controls light intensities by utilizing alignment of a liquid crystal, a plasma display panel (referred to as a PDP) in which ultraviolet rays in plasma discharge energize a fluorescent material to emit light, a field emission display (referred to as an FED) in which an electron beam emitted from a field emission type electron emission element energizes a fluorescent material to emit light, and a surface-conduction electron emitter display (referred to as an SED), as a kind of FED, which uses a surface-conduction electron emission element.
Generally, the FED includes a front substrate and a back substrate which are oppositely arranged while being spaced from each other by a predetermined gap. Those substrates are bonded together at the peripheral portions in a state that a rectangular frame-like side wall is interposed therebetween, thereby forming a vacuum envelope. A fluorescent screen is formed on the inner surface of the front substrate. A number of electron emission elements as electron emission sources for exciting the fluorescent material to emit light are provided on the inner surface of the back substrate.
A plurality of support members are arranged between the back substrate and the front substrate in order to support the atmospheric load applied to those substrates. A potential of the back substrate is substantially equal to an earth potential, and an anode voltage is applied to the fluorescent surface. An image is displayed in such a manner that red, green and blue fluorescent materials constituting the fluorescent screen are irradiated with electron beams emitted from a number of electron emission elements, thereby to light.
Those types of display devices may be reduced in thickness to about several millimeters. The size and thickness reduction could be achieved as compared to the CRT, currently used for televisions and computers.
In the case of the FED, it is necessary to evacuate the inside of the envelope. Also in the case of the PDP, the envelope is evacuated and then filled with discharge gas. In Jpn. Pat. Appln. KOKAI Publication No. 2001-229825, there is a proposal of means for evacuating an envelope. In the proposal, the final step of assembling a front substrate and a back substrate, which constitute the envelope, is carried out in a vacuum vessel.
In this method, the front and back substrates, which are placed in the vacuum vessel, are sufficiently heated. This is done to suppress the discharging of gas through the inner wall of the envelope, which is a major cause of degrading the degree of vacuum in the envelope.
When the front and back substrates are cooled and the degree of vacuum in the vacuum vessel is satisfactorily increased, a getter film to improve the degree of vacuum in the envelope and maintain the improved one is formed on a fluorescent screen. Following this, the front and back substrates are heated again up to a temperature at which a sealing material melts. The front and back substrates are cooled in a state that those are assembled at a predetermined position until the sealing material is solidified.
In the vacuum envelope manufactured by such a method, the sealing step and the vacuum sealing step are carried out at one time, there is no need of spending such a time taken as in the case of exhausting the envelope, and an extremely satisfactory degree of vacuum can be obtained.
A side wall of the envelope is formed with a glass frame as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2002-319346. The glass frame, when it is relatively small, is manufactured by directly press-molding molten glass or directly cutting it out of a thin sheet glass of large size.